BEIJING -- A new government survey of China's environmental problems has shown that water pollution levels in 2007 were more than twice the official estimate, largely because agricultural waste was ignored.

The data, presented Tuesday by Vice Environment Protection Minister Zhang Lijun, revives questions about the quality of China's official statistics and the effectiveness of a government push for cleaner growth after decades of unbridled expansion.

The first national census on pollution sources found that discharge of chemical oxygen demand, or COD -- a measure of water pollution -- in wastewater was 30.3 million tonnes, Zhang said. The government had said in an official paper published two years ago that 2007 was the first year it managed to reduce water pollution, with COD falling 3 percent to 13.8 million tonnes.

The census has been years in the making, in part because it was extremely comprehensive but possibly also because the contents include painful revelations such as this one.

Zhang played down the difference, saying it was explained by the survey's expanded scope, the inclusion of agricultural sources of wastewater and different calculation methods.

Figures for other pollutants did not suggest widespread fiddling of data.

Acid-rain-causing sulfur dioxide emissions, for example, were pegged at 23.2 million tonnes by the census and had been estimated at 24.7 million tonnes in the earlier data.

Zhang said the survey gives China a better idea of its challenges and is a sign of the country's commitment to shifting its economic model, which he added should allow China to cap pollution growth at an earlier stage of development than Western nations.

Activists welcomed the effort to compile a more comprehensive picture of the country's pollution problem but called for greater transparency.

"It appears that the comprehensive pollution data from the census has not been made accessible to the public," Greenpeace campaign director Sze Pang Cheung said. "We urge the government to immediately establish a strong platform through which the public could easily access a wide range of pollution data."